1. Field of the Invention
This invention relates to chemical mechanical polishing, used in a variety of technologies to produce planarized surfaces and to polish thin films, and in particular to the semiconductor wafer industry. More particularly, the invention relates to the detection of an endpoint for the polishing process using optical endpoint detection apparatus, and the calibration of these apparatus.
2. Description of the Related Art
Chemical mechanical polishing (CMP) has emerged as a critical technology in the manufacture of electronic devices on semiconductor wafers, particularly for the fabrication of devices that have critical dimensions smaller than 0.5 microns. CMP is used at various stages in the fabrication of semiconductor electronic devices on semiconductor wafers, and is generally used to either remove excess material deposited on the wafer surface, or to planarize the wafer surface, or both. The removal of excess material at various points in the process is necessary to produce the electrical interconnects required in the completed device. Also, planarization at various points in the process is necessary to ensure accurate and precise development of the structure of the electronic devices being fabricated in the wafer surface.
An important aspect of CMP is endpoint detection (EPD). Endpoint detection is the determination of when to terminate polishing. Many users prefer EPD systems that are xe2x80x9cin-situxe2x80x9d, that predict or detect the endpoint during the polishing process.
One of the techniques for EPD is the use of optical systems. For example, an optical EPD system is disclosed in U.S. Pat. No. 5,433,651. In this patent, an optical fiber carrying a light signal transmits the signal through a window in the platen of a rotating CMP tool to interrogate the wafer surface and a reflected optical signal is analyzed to determine whether the endpoint has been reached.
Another approach entails monitoring absorption of particular wavelengths of the infrared spectrum of a light beam incident on the rear side (i.e. side not being polished) of a wafer that is being polished. Thus, the beam passes through the wafer from the non-polish side of the wafer to the other side to detect a thin film on the surface being polished. Changes in the absorption of infrared light, within narrow well-defined spectral windows, correspond to changing thickness of specific types of films on the wafer surface being polishing. An example of this approach is disclosed in U.S. Pat. No. 5,643,046.
In general, in a semiconductor fabrication facility (xe2x80x9cfabxe2x80x9d) a plurality of spectrometers will be used if optical EPD is the method of detecting endpoint. At least one spectrometer would be associated with each CMP tool. In these systems, a light source generally provides light through a optical fiber to the surface of a wafer. Light that is either reflected from or transmitted through the wafer is received in a second fiber, and transmitted to a spectrometer, where the light is channeled into various components based on wavelength. As received from vendors, the wavelength per channel and the linearity of the wavelength per channel is generally not always identical from one spectrometer to another within the same fab and the degree of variation poses limitations. Thus, for example, spectrometers are not generally interchangeable within a fab, and may not accurately predict endpoint if they are so interchanged. Further, if the fab uses a technique that permits the spectrometer to compare input light with stored data from a standard wafer, then each spectrometer would have to be calibrated separately, and differently, for that standard wafer.
This summary of the invention section is intended to introduce the reader to aspects of the invention and is not a complete description of the invention. Particular aspects of the invention are pointed out in other sections hereinbelow and the invention is set forth in the appended claims, which alone demarcate its scope.
The invention provides calibrated spectrometers in a multi-spectrometer system, such as found in a fab, that are each configured with respect to wavelength and light intensity for the optical detection of endpoint during chemical mechanical polishing of semiconductor wafers, flat panel displays, lenses, and other workpieces that undergo polishing where endpoint detection is an issue.
In one aspect of the invention, a spectrometer is calibrated by selecting a filter slide having a predetermined light transmittance or reflectance variation with location (e.g. angular or linear displacement) on the slide. Light is incident on locations on the filter slide, and this incident light is either transmitted or reflected. Transmitted or reflected light is received by a spectrometer, and the wavelength measured is compared with the known wavelength that corresponds to its location on the slide. The spectrometer is calibrated by normalizing the wavelength readings obtained at various locations on the slide with the known readings dictated by the reference slide. This normalizing may be carried out by any of a number of mathematical techniques, some of which are discussed herein. A similar normalization technique is also carried out for light intensity reading normalization, in accordance with the invention.
When all spectrometers in a multi-spectrometer system have been calibrated using the normalization techniques of the invention for wavelength and intensity, then a single test workpiece may be polished, and continuously optically monitored during polishing with any one of the calibrated spectrometers. Spectral measurements obtained from the monitoring are normalized, using the same normalization function/factors used in the calibration of the spectrometer. The endpoint for polishing may be determined based on the normalized data, and the normalized endpoint spectral signature in terms of wavelength and intensity may be determined. This normalized information may then be digitally stored and utilized on any of the other spectrometers for endpoint detection, since each of these have been normalized (by its own unique normalization factors) to a common standard for both wavelength and intensity. Absent such normalization, each spectrometer might have to be calibrated by polishing a test workpiece for each spectrometer separately and continuously monitoring that test workpiece during polishing to develop a spectral signature for endpoint determination.
During polishing of workpieces, each spectrometer monitors surface spectral data, and converts these via its unique normalization factors to normalized values that are then compared with the normalized stored spectral data from the test piece. Once measured data (after normalization) approaches the endpoint set for the test piece within a predetermined degree of difference the endpoint of CMP has been reached and polishing may be manually or automatically terminated.
Thus, the invention eliminates the need for the polishing of large numbers of test workpieces for instrument calibration, and permits the potential interchangeability of one spectrometer for another, in a multi-spectrometer system, without need for further recalibration.